Band saw machines include Vertical band saw machines and horizontal band saw machines. Each of the vertical and horizontal band saw machines includes a band saw blade guide device configured to hold and guide the non-toothed section of the band saw blade from both sides at a location close to a workpiece in the process of cutting the workpiece with the band saw blade. The band saw blade guide device includes: a back portion supporting member configured to support and guide the back portion of the band saw blade; a fixed insertion block and a movable insertion block configured to hold the non-toothed section of the band saw blade therebetween to guide the non-toothed section; and an insertion block pressing unit configured to press the movable insertion block toward the fixed insertion block.
The conventional insertion block pressing unit generally employs either a configuration in which the movable insertion block is pressed by the rotation of an eccentric cam or a configuration in which the movable insertion block is pressed using a small hydraulic cylinder. The configuration using the eccentric cam is designed to always move the movable insertion block toward the fixed insertion block up to the same position. For this reason, when the clearance between the fixed insertion block and the movable insertion block is set at an appropriate clearance corresponding to the a band saw blade with a thin non-toothed section, the configuration has a problem of holding another band saw blade with a thick non-toothed section too firmly between the fixed insertion block and the movable insertion block. Conversely, when the clearance has been set corresponding to a band saw blade with a thick non-toothed section, a large clearance occurs between another band saw blade with a thin non-toothed section and each of the insertion blocks. In this case, the band saw blade tilts, so that cutting deviation is likely to occur.
In the configuration in which the movable insertion block is pressed using the small hydraulic cylinder, the non-toothed section of the band saw blade is always firmly held between and guided by the fixed insertion block and the movable insertion block regardless of the thickness of the non-toothed section of the band saw blade. For this reason, this configuration has problems that the two insertion blocks wear out very much, and that the power for driving the band saw blade is large.
Against this background, a technique is employed in which the movable insertion block is slightly returned after the non-tooth section of the band saw blade is firmly held between the fixed insertion block and the movable insertion block. For example, an aspect disclosed in Patent Document 1 uses this technique.
In the configuration described in Patent Document 1 above-mentioned, as shown in FIG. 1, a; the non-toothed section of a band saw blade 1057 is firmly held between a fixed insertion block 1055 and a movable insertion block 1051 by moving the movable insertion block 1051 toward the fixed insertion block 1055 by a hydraulic cylinder 1053 via a lever 1054; and thereafter, the movable insertion block 1051 is slightly returned by sucking the pressure oil in the hydraulic cylinder 1053 into a small cylinder 1061 in a booster 1059. With this configuration, the clearance between the fixed insertion block 1055 and the movable insertion block 1051 can be kept at an appropriate clearance regardless of the thickness of the non-toothed section of the band saw blade 1057.
Here, assume that: the clearance is regarded as appropriate when a value of the clearance dimension is 0.05 mm, which is obtained by subtracting the thickness dimension of the band saw blade 1057 from a gap dimension between the fixed insertion block 1055 and the movable insertion block 1051; and that the invention described in Patent Document 1 mentioned above is applied to an actual band saw machine. If the lever ratio of the lever 1054 is 1:2, the piston of the hydraulic cylinder 1053 needs to be retracted by 0.1 mm in order to retract the movable inserting block 1051 by 0.05 mm after the band saw blade 1057 is firmly held between the fixed insertion block 1055 and the movable insertion block 1051.
In this case, if the diameter and main pressure of the hydraulic cylinder 1053 are, respectively, 10 mm and 50 kgf, the clamping force of the movable insertion block 1051 is 0.785 cm2 (the cross-sectional area of the cylinder)×50 kgf/cm2 (the main pressure)×2 (the lever ratio)=78.5 kgf. From this, the amount of oil needed to retract the piston of the hydraulic cylinder 1053 by 0.1 mm is 0.01 cm×0.785 cm2=0.00785 cm3=0.785 cc. As a result, approximately 0.8 cc of pressure oil needs to be discharged from the hydraulic cylinder 1053.
Although the minuscule amount of pressure oil, i.e., the approximately 0.8 cc is discharged after the band saw blade 1057 is held and pressed with a pressure of the 78.5 kgf as described above, the band saw blade 1057 may not be released from being held or pressed in some cases, with only the pressure decreasing. Specifically, the pressure oil used for the hydraulic system generally contains minuscule amounts of gases including air, and it is highly likely that the minuscule amounts of gases merely increase in volume. For this reason, the configuration using the hydraulic cylinder has the problem in actual use.